Wind turbine blades have a typical service life of about 20 years. During that time, wind turbine blades are subjected to a variety of forces, including static and dynamic lift and inertial and drag loads. Further, wind turbine blades must endure these forces over a wide range of environmental conditions, such as temperature extremes, ultraviolet light, precipitation (rain, snow, sleet, and hail), and bird strikes. Wind turbine blades must be specially constructed so that they withstand the myriad forces and conditions over their 20 year service life by combining low weight and low rotational inertia with high rigidity and resistance to fatigue and wear.
A typical wind turbine blade is constructed of layers of an outer skin supported by a primary spar. For example, and as illustrated in FIGS. 1-3, a wind turbine blade 100 has a turbine tip 102 and an opposing turbine root 104. Extending between the tip 102 and the root 104 are a spar cap 106 and a shear web 108. The shear web 108 serves as the main structural support within a turbine blade 100. The spar cap 106 is a glass portion running the length of the turbine blade coincident with the shear web 108 and it serves to take the tensile load of the blade 100.
Turbine blades, such as turbine blade 100, are formed in shells. For example, a first shell 105a extending from a leading edge 114 and a trailing edge 116 and including a suction surface 118 is positioned within a mold. This first shell 105a includes areas that include fiber-reinforced material 110 and other areas that include a core material 112. The core material portion may be composed of, for example, foam, balsa wood, or engineered core materials. Foam cores may include, for example, polyvinyl chloride (PVC), urethane, or polyethylene terephthalate (PET). Balsa wood has low cost, good shear properties, but a higher weight than the other core materials. Examples of engineered core materials include Webcore TYCOR® and NexCore™
The first shell 105a is placed in the mold such that the suction surface 118 is against the mold and a surface B is exposed. A second shell 105b extending from the leading edge 114 to the trailing edge 116 and including a pressure surface 120 is placed in a second mold such that the pressure surface 120 is against the mold and a surface B is exposed. As with the first shell 105a, the second shell 105b includes areas that are primarily glass and other areas that include the core material 112.
The shells 105a, 105b may be applied as multiple thin layers. Each of the layers may be a fiber-resin matrix. The layers of shells 105a, 105b may be formed of E-glass fiber or a carbon fiber bonded with a composite resin. Other potential composite materials include graphite, boron, aramid, such as KEVLAR®, and other organic materials and hybrid fiber mixes that can form reinforcing fibers. The reinforcing fibers may be in the form of a continuous strand mat (CSM), woven, or unidirectional mat (UNI). There are two main classes of polymer resin matrices—thermoset resins and thermoplastic resins. Thermoset resins include epoxy, phenols, bismaleimide, and polyimide, while thermoplastic resins include polyamide such as NYLON®, polysulfone, polyphenylene sulfide, and polyetheretherketone (PEEK). The matrix holds the fibers in place and, under an applied load, deforms and distributes stress to the fibers.
The composite layers may be formed into laminate or sandwich structures. Laminate structures include successive layers of composite materials bonded together. Sandwich structures include a low-density core between layers of composite materials.
The strengthening effect of the fiber reinforcements found in the layers of shells 150a, 105b depends on the percentage of fibers (also known as the fiber volume fraction), the types of fibers, the orientation of the fibers with respect to the direction of the loads, and the bond strength between the fibers and the matrix.
Sometimes, during construction of the molded shells 105a, 105b a defect can occur that reverberates throughout the multiple layers of either the first shell 105a or the second shell 105b or at the bond site between the spar cap 106 and the first shell 105a. The typical areas of concern for a defect are at the leading edge 114, the trailing edge 116, and near the spar cap 106. These three areas each carries the tensile loading of the blade 100, and so any bending of the fibers in a span-wise direction 124 in these sections reduces the strength of the fibers. For example, a burr or other anomaly may protrude from the B surface of the first shell 105a at the spar cap 106 or from the B surfaces of either shells 105a, 105b at the leading edge 114 or the trailing edge 116.
Depending upon where the anomaly is located in the first or second shells 105a, 105b, the anomaly may be visible in surfaces B while in the mold. Since surfaces B are interior surfaces of the blade 100 once it is constructed, any anomaly that may have been visible on either of surfaces B would no longer be visible after the first and second shells 105a, 105b are glued together to form the blade 100. Since the suction surface 118 and the pressure surface 120 are against the molds, these sides will not move and will contour the molds. Thus, any defect or wrinkle can only be detected from the B surfaces. The depth of the anomaly within the shells 105a, 105b determines whether the anomaly is visibly detectable on the B surfaces. So, if the anomaly is located nearer to a leading edge 114, sufficient layers of material within the first or second shells 105a, 105b may be applied over the anomaly to render the anomaly invisible to an external inspection of surface B prior to gluing. Conversely, if the anomaly is located nearer to a trailing edge 116, there may not be enough layers of material within the first or second shells 105a, 105b to render the anomaly invisible to an external inspection of surface B prior to gluing.
Anomalies in the construction of wind turbine blades, whether visible to external inspection prior to gluing or not, affect the strength of the turbine blade. Some anomalies create such an adverse effect on the strength of the turbine blade that the turbine blades are considered out of spec and do not pass inspection. In such instances, the turbine blades must either be sent back to be corrected or scrapped.
Current inspection techniques include an external inspection prior to gluing the two shells 105a, 105b together, coupled with a look-up table. A visual inspection of the shells prior to gluing may show an externally visible defect. Such an externally visible defect can be measured for its length (L) and its height (ae). Current look-up tables include strength reductions for respective external defect aspects (L/ae).
Current inspection techniques are all external and take place prior to gluing, and thus the only parameters of a defect that are measured are the external defect aspects of length (L) and height (ae). Where the defect is located within a wind turbine blade affects the ultimate theoretical strength of the wind turbine blade, and current inspection techniques are not able to ascertain locations within a wind turbine blade. Additionally, if the defect is sited such that it is not visible from an external inspection of surfaces B prior to gluing, current inspection techniques will be unable to detect a defect, and thus may pass wind turbine blades that are strength compromised.
What is desired is an inspection technique that is capable of determining more parameters of a defect and is capable of determining a defect exists even though it is not externally visible.